The present disclosure relates to a sheet transport device that is used in an image forming apparatus such as a digital copying machine or a laser printer and that transports a sheet or an original document. More particularly, the present disclosure relates to a sheet separation transport mechanism that separates and transports a stack of sheets one by one.
Conventionally, in a paper feed device that is incorporated in a copying machine or the like using an electrophotographic process, transportation is generally performed with the uppermost surface of a stack of sheets pressed onto a paper feed roller. Then, a sheet separation transport mechanism that uses a separation pad pressed onto the paper feed roller to separate and transport only a sheet on the uppermost surface of the stack of sheets.
For example, a paper feed device is known in which, in sheet guide shoots placed on both sides of a division member that divides and then separates and feeds a stack of sheets one by one, shoot ribs higher than the upper surface of the division member are formed, the back surface of a sheet transported between a paper feed roller and the division member is slidingly brought into contact with the shoot ribs to produce a resistance force in the paper feed direction of the sheet and thus the simultaneous feeding of a plurality of sheets is reduced.
A paper feed device is also known in which, in the paper feed device separating and feeding, one by one, sheet members between a paper feed roller and a division member, an inclination angle of the surface of a stack member where the sheet members are stacked with respect to the surface of the division member in contact with the sheet members is made changeable and thus it is possible to adjust, according to the rigidity of the sheet member, the entry angle of the sheet member from the stack member into the division member.
The sheet separation transport mechanism described above is required to have the function of dividing a plurality of sheets one by one and the function of transporting the sheet to the subsequent transport roller. When a higher priority is given to the division function, the frictional force of the separation pad is increased and the transport load is increased, and thus the division function is enhanced; however, especially in heavy paper, the transport load is excessively increased, with the result that the sheet is disadvantageously prevented from being transported to a nip portion (main separation portion) between the paper feed roller and the separation pad.
On the other hand, in order for the function of transporting the heavy paper to be enhanced, the transport load of the division member is lowered such as by decreasing the angle of the separation pad, selecting a separation pad having a low frictional coefficient or reducing a spring load pressing the separation pad; in this case, the division function is degraded.
In particular, when, in an operation of feeding paper from a paper feed cassette and an operation of feeding paper from a manual tray placed above the paper feed cassette, separation transport is performed using the same paper feed roller and separation pad, since the entry angle of the sheet from the manual tray into the separation pad is increased, it is more likely that a sheet, such as heavy paper, that has a high elasticity is disadvantageously stopped in front of a main separation portion.